Thin film lithium niobate integrated optical engine for fiber-optic gyroscope

Abstract

The invention provides a thin-film lithium niobate integrated optical engine for a fiber-optic gyroscope. The thin-film lithium niobate integrated optical engine comprises a substrate, a lower cladding, a lithium niobate thin-film waveguide core layer and an upper cladding, the lithium niobate film waveguide core layer comprises an incident unit, a polarizing unit, a coupling unit, a beam splitting unit, a spot size conversion unit, a phase modulation unit and an emergent unit; the input unit and the polarization unit are gas-phase proton exchange waveguides and naturally have a polarization function; the coupling unit and the beam splitting unit are integrated into a whole, the structure is 2 * 2MMI, and a gas-phase proton exchange process is adopted for preparation; the phase modulation region adopts a lithium niobate film ridge waveguide structure, and coplanar traveling wave electrodes are arranged to realize an electro-optical modulation function; and spot-size conversion structures are additionally arranged between the gas-phase proton exchange waveguide and the ridge waveguide as well as between the ridge waveguide and the output optical fiber, so that mode field matching is realized. According to the embodiment of the invention, the lithium niobate film platform is utilized, and gas-phase proton exchange is combined with a ridge waveguide etching process, so that multiple functions of polarization, coupling, beam splitting, modulation and the like are integrated, the problem of optical coupling loss is solved, and the purposes of miniaturization and low cost of future fiber-optic gyroscopes are achieved.

Description

Technical Field

[0001] The present invention relates to the field of fiber optic gyroscopes, and in particular to a thin-film lithium niobate integrated optical engine for fiber optic gyroscopes. Background Art

[0002] Fiber optic gyroscopes are used to sense the angular rate of rotation of sensitive carriers. They are one of the core sensors of inertial navigation systems and are widely used in modern aviation, aerospace, navigation and defense industries. At the same time, they are also widely used in civilian fields such as autonomous driving, earthquake monitoring, and mineral exploration. Traditional fiber optic gyroscopes usually use discrete optical components to integrate light sources, detectors, polarizers, couplers, Y-waveguide phase modulators, and fiber ring components by fiber fusion to make fiber optic gyroscope prototypes. Y-waveguide phase modulators are generally made of lithium niobate materials through proton exchange technology. The refractive index difference between the waveguide core layer and the cladding is very small. The device is large in size and weight, resulting in large size, difficult integration, and high cost of fiber optic gyroscope prototypes. Generally, fiber optic gyroscope prototypes are not less than 60mm, which is difficult to meet the future military and civilian fields. The demand for small-volume, low-cost fiber optic gyroscopes. Using photon integrated chips to replace traditional fiber discrete devices and taking advantage of the advantages of large-scale mass production of integrated optical lithography processes to increase the miniaturization and low-cost of fiber optic gyroscopes is an important direction for the development of fiber optic gyroscopes.

[0003] Lithium niobate thin film optical waveguide has the high electro-optic coefficient, acousto-optic coefficient, and good nonlinear effect of traditional lithium niobate waveguide, and can realize highly integrated integrated optical circuits, thus becoming the most promising optoelectronic device platform. At the same time, lithium niobate thin film combined with proton exchange process has a natural polarizing function, so it can replace the polarizer device, further reducing the size and cost. However, traditional proton exchange waveguides generally use liquid-phase proton exchange process, and the prepared waveguide has low refractive index contrast, weak light restriction ability, and large lattice damage. The use of improved gas-phase proton exchange process can make up for the above shortcomings, further reduce the transmission loss of the waveguide, and match the mode field size of the input optical fiber, without the need to add a mode spot conversion structure. However, compared with proton exchange waveguides, lithium niobate thin film ridge waveguides can retain the good electro-optical properties of lithium niobate, have a higher refractive index contrast, and the waveguide size can be smaller, making it easier to realize chip miniaturization and integration.

[0004] In order to give full play to the respective advantages of proton exchange waveguides and ridge waveguides and realize the functions of fiber optic gyroscope polarization, coupling, and beam splitting, waveguides with different structures need to be integrated on the same lithium niobate thin film chip. Considering the mismatch of mode fields among proton exchange waveguides, ridge waveguides, and output optical fibers, a mode spot conversion structure needs to be added. How to integrate waveguides with different structures on the same lithium niobate thin film chip to meet the development needs of small size and low cost while ensuring accuracy is an urgent problem to be solved. Summary of the invention:

[0005] To solve the above problems, the present invention provides a thin-film lithium niobate integrated optical engine for a fiber optic gyroscope, which achieves the development goals of miniaturization and low cost of the fiber optic gyroscope and is conducive to the mass production of integrated fiber optic gyroscopes.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] As attached Figure 1 As shown, a thin-film lithium niobate integrated optical engine for a fiber optic gyroscope includes, from bottom to top:

[0008] Substrate 01, lower cladding 02, lithium niobate thin film waveguide core layer 03, upper cladding 10;

[0009] The lithium niobate thin film waveguide core layer includes an incident unit 04, a polarizing unit 05, a coupling unit 0601, a beam splitting unit 0602, a mode spot conversion unit 07, a phase modulation unit 08, and an output unit 09;

[0010] The incident unit 04 is connected to the light source to couple the incident light to the polarizing unit 05. Both the incident unit 04 and the polarizing unit 05 are prepared by gas-phase proton exchange process. Compared with the traditional liquid-phase proton exchange, gas-phase proton exchange does not require annealing and is less damaging to the crystal structure. The mode field of the gas-phase proton exchange waveguide is similar in size to the mode field of the incident optical fiber, and the degree of matching with the mode field of the optical fiber is very high. No additional mode spot conversion structure is required. In addition, the gas-phase proton exchange has a natural polarizing function and can only transmit TE and TM modes. Therefore, the functions of the incident unit 04 and the polarizing unit 05 can be integrated;

[0011] As attached Figure 2 As shown, the coupling unit 0601 and the beam splitting unit 0602 are 2×2 MMI structures, which are also prepared by gas phase proton exchange method. The incident light is input from port 1 and output from ports 3 and 4. The output light splitting ratio of the two ports is 1:1 by designing the structural dimensions of the MMI. Instead of the Y branch structure, the coupling unit 0601 and the beam splitting unit 0602 are integrated into one, which is more conducive to integration and further reduces the size of the device.

[0012] The first and second input waveguides of the 2×2MMI have the same structure as the third and fourth output waveguides and are distributed symmetrically in the multimode waveguide region;

[0013] The length L of the MMI multimode interference region is:

[0014]

[0015]

[0016]

[0017] For TE light, σ=0, for TM light, σ=1, W is the width of the multimode interference zone, nr is the core refractive index, ns is the cladding refractive index, λ0 is the wavelength of light, and the positions of the two input and output waveguides of the 2×2MMI are ±W / 6; the position of the first double self-image is at Lπ / 2.

[0018] The 2×2 MMI is also used to combine the two signal lights after re-modulation to generate an interference signal, which is emitted from port 2 of the 2×2 MMI and coupled to the photodetector to realize signal monitoring. Figure 6 As shown, it is the light field transmission diagram of 2×2MMI.

[0019] As attached Figure 1 As shown, the phase modulation unit 08 includes a lithium niobate film ridge waveguide 0801 and a traveling wave electrode 0802. The lithium niobate film ridge waveguide 0801 is prepared on the lithium niobate film 03 by using an ICP etching method, and the etching depth and waveguide size are obtained by simulation using lumerical software;

[0020] The traveling wave electrodes 0802 are distributed on both sides of the modulation arm of the ridge waveguide 0801. The electrode material is gold, and it is a coplanar waveguide push-pull structure. The middle of the two modulation arms is the signal electrode, and the two sides are ground electrodes. When an electrical signal is applied, the two single-polarized lights are modulated to achieve the electro-optical modulation function.

[0021] The structural parameters of the electrode width, electrode thickness and electrode spacing of the traveling wave electrode 0802 are obtained by HFSS simulation, which meet the requirements of impedance matching, microwave phase velocity matching and light wave group velocity matching, and meet the requirements of low RF attenuation;

[0022] As attached Figure 3 As shown, the pattern conversion unit 07 is used for mode field matching between gas-phase proton exchange and ridge waveguide; the pattern converter 1 is made of silicon oxynitride, and the refractive index is regulated between lithium niobate and silicon dioxide; the structure is a double-layer tapered gradient structure, the wide end of the lower tapered waveguide 0703 is butted with the front proton exchange waveguide, the two waveguides have the same thickness, the width is wider than the proton exchange waveguide, and the tip part is covered with the lithium niobate film ridge waveguide 0801; the wide end of the upper tapered waveguide 0702 is consistent with the width of the wide end of the lower tapered waveguide 0703, and the waveguide tip ends at the lithium niobate film ridge waveguide 0801; the left side of the upper tapered waveguide 0702 extends to the proton exchange waveguide and covers the proton exchange waveguide;

[0023] Preferably, the pattern conversion structure 07 is formed by plasma enhanced chemical vapor deposition (PECVD), and the reaction equation is: the refractive index of silicon oxynitride is regulated by controlling the ratio of N2O and SiH4, and the double-layer conical structure is prepared by inductively coupled plasma etching (ICP) and a two-step photolithography process;

[0024] As attached Figure 4 As shown, the output unit 10 also includes a mode spot conversion structure for coupling between the lithium niobate film ridge waveguide 0901 and the output optical fiber; the mode spot conversion structure adopts a double-layer tapered gradient waveguide 0902, 0903 structure, and cooperates with the silicon oxynitride waveguide 0904 at the end to achieve the amplification of the light spot size; the gradient slope of the bottom cone, i.e., the deep etching area, is smaller than that of the top cone, i.e., the shallow etching area, and the top cone is cut off when the bottom cone is narrowed to a range of 4-6um;

[0025] The end of the emission unit 10 is fused with the optical fiber coil via a pigtail.

[0026] As attached Figure 1 As shown, the substrate material 01 is silicon, the material of the lower cladding 02 is silicon dioxide, the material of the lithium niobate thin film waveguide core layer 03 is lithium niobate thin film, and the material of the upper cladding 10 is silicon dioxide with a refractive index less than a preset refractive index threshold.

[0027] The advantages and positive effects of the present invention are:

[0028] By adopting thin-film lithium niobate integrated optical engine and combining gas-phase proton exchange with ridge waveguide, passive devices such as beam splitters, polarizers, couplers, and mode spot converters are integrated on-chip to replace traditional optical fiber discrete devices, thus achieving the development goal of small size and low cost of optical fiber gyroscope. Description of the drawings:

[0029] Figure 1 A three-dimensional schematic diagram of an embodiment of the present invention;

[0030] Figure 2 is a two-dimensional top view schematic diagram of a 2×2 MMI in an embodiment of the present invention;

[0031] Figure 3 is a two-dimensional top view schematic diagram of a pattern spot conversion structure in an embodiment of the present invention;

[0032] Figure 4 is a two-dimensional top view schematic diagram of an emission unit in an embodiment of the present invention;

[0033] Figure 5 A complete process flow chart for preparing an embodiment of the present invention;

[0034] Figure 6 Schematic diagram of transmission of a light field in a waveguide according to an embodiment of the present invention; DETAILED DESCRIPTION

[0035] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0036] As attached Figure 1 As shown, a thin-film lithium niobate integrated optical engine for a fiber optic gyroscope includes, from bottom to top:

[0037] Substrate 01, lower cladding 02, lithium niobate thin film waveguide core layer 03, upper cladding 10; the substrate 01 is made of silicon with a thickness of 500um; the lower cladding 02 is made of silicon dioxide with a thickness of 4.7um; the lithium niobate thin film waveguide core layer 03 is made of lithium niobate with an x-cut, and the film is prepared by ion implantation and wafer bonding with a thickness of about 600nm; the upper cladding 10 is made of silicon dioxide with a thickness of 800nm, and the refractive index is less than a preset refractive index threshold.

[0038] The lithium niobate thin film waveguide core layer 03 includes an incident unit 04, a polarizing unit 05, a coupling unit 0601, a beam splitting unit 0602, a mode spot conversion unit 07, a phase modulation unit 08, and an output unit 09;

[0039] The incident unit 04 is connected to a light source. The wavelength of the light source in this embodiment is 1310nm. The incident light is coupled to the polarizing unit 05. Both the incident unit 04 and the polarizing unit 05 are prepared by gas-phase proton exchange process. Compared with the traditional liquid-phase proton exchange, gas-phase proton exchange does not require annealing and is less damaging to the crystal structure. The mode field of the gas-phase proton exchange waveguide is similar to the mode field of the incident optical fiber. The wire width of the gas-phase proton exchange waveguide is 3um, the refractive index contrast is 0.05, and the mode field diameter is about 3.5um. The degree of matching with the mode field of the optical fiber is very high, and no additional mode spot conversion structure is required. In addition, the gas-phase proton exchange process has a natural polarizing function and can only transmit TE and TM modes. This embodiment uses an x-cut lithium niobate crystal, which can only support TE mode transmission and can replace the polarizer structure, which is conducive to chip miniaturization.

[0040] As attached Figure 2As shown, the coupling unit 0601 and the beam splitting unit 0602 are structured as 2×2MMI, which are also prepared by gas phase proton exchange method. The incident light is input from port 1 and output from port 3 and port 4. The two-port output light splitting ratio of 1:1 is achieved by designing the structural dimensions of the MMI. After a large number of simulation calculations using lumerical software, the structural parameters of the MMI are obtained: the center spacing g between the two input waveguides is 5um, the input waveguide is widened from 3um line width to Wt of 4um, the widening length Lt is 100um, the length L of the multimode interference zone is 210um, the width W is 14um, and the input and output waveguides are symmetrical. The 2×2MMI is also used to combine the two signal lights after secondary modulation to generate an interference signal, which is emitted from port 2 of the 2×2MMI and coupled to the PD detector to realize signal monitoring. The light field transmission diagram is shown in Figure 6 shown.

[0041] As attached Figure 1 As shown, the phase modulation unit 08 includes a lithium niobate thin film ridge waveguide 0801 and a traveling wave electrode 0802. The lithium niobate thin film ridge waveguide 0801 is prepared by ICP etching on a lithium niobate thin film. The ICP etching technology has the advantages of fast etching rate, high selectivity, high anisotropy, small etching damage, good uniformity over a large area, and smooth etching sidewalls and surfaces. The etching depth and waveguide size are obtained by simulation using lumerical software, and the etching depth is 300nm and the waveguide line width is 1.2um;

[0042] The traveling wave electrodes 0802 are distributed on both sides of the modulation arm of the ridge waveguide 0801. The electrode material is gold, and it is a coplanar waveguide push-pull structure. The middle of the two modulation arms is the signal electrode, and the two sides are ground electrodes. When an electrical signal is applied, the modulation electric fields of the two modulation arms are opposite. Under the same modulation voltage, the phase difference generated by the push-pull structure is twice that of the single-arm modulation, thereby improving the modulation efficiency and realizing the electro-optical modulation function.

[0043] The structural parameters of the traveling wave electrode 0802, including electrode width, electrode thickness and electrode spacing, are obtained by HFSS simulation, which meets the requirements of impedance matching, microwave phase velocity matching and light wave group velocity matching, and low RF attenuation. After a large number of simulation calculations, the structural parameters of the traveling wave electrode are as follows: electrode spacing is 5um, signal electrode width is 10.5um, electrode thickness is 1.2um, and the length of the two modulation arms is about 8mm.

[0044] As attached Figure 3As shown, the pattern spot conversion unit 07 is used for mode field matching between the above-mentioned gas phase proton exchange and the ridge waveguide; the mode field diameter of the gas phase proton exchange waveguide is about 3.5um, and the mode field diameter of the ridge waveguide is less than 1um. The direct coupling of the two will produce a large coupling loss. Therefore, it is necessary to add a pattern spot conversion structure to match the mode fields of the two. In this embodiment, the pattern spot conversion structure material is silicon oxynitride, and the refractive index is set to 1.8. It is connected to the output end face of the proton exchange waveguide. The lower tapered waveguide 0703 has a wide end width of 3um, a tip width of 2um, a length L2 of 20um, a taper change rate of 35nm / um, and a thickness of 0.6um. The tip of this tapered waveguide is partially deposited on the first input waveguide, and the coverage length L3 is 10um. The width of the wide end of the upper tapered waveguide 0702 is the same as the wide end of the proton exchange waveguide, which is 3um, and the thickness is 0.3um. The tip ends at the first input waveguide on the right, and the width is 200nm. In addition, the left side of the upper tapered waveguide 0702 extends to the proton exchange waveguide and covers the upper layer of the proton exchange waveguide. When preparing the pattern conversion structure, the 0.6um lithium niobate film between the proton exchange waveguide and the MMI is first etched using the ICP technology. After the etching is completed, the silicon oxynitride is deposited and prepared using the PECVD method at 300°C in a mixed gas environment of NH3, N2O, and SiH4. The reaction equation is: The refractive index of silicon oxynitride is controlled by controlling the ratio of N2O and SiH4, and the double-layer tapered pattern conversion structure is formed using the ICP etching process.

[0045] As attached Figure 4 As shown, the output unit 09 also includes a mode spot conversion structure for matching the mode field between the lithium niobate thin film ridge waveguide 0901 and the output optical fiber. The ridge waveguide mode field diameter is <1um, and the optical fiber mode field diameter is about 6.5um. The mode spot conversion structure adopts a double-layer conical gradient waveguide structure, which cooperates with the silicon oxynitride waveguide at the end to achieve the amplification of the spot size. The manufacturing process of silicon oxynitride is similar to the above method, and the structural parameters are: L4=100um, L5=200um, ω1=0.12um, ω2=0.15um, and the cross-sectional size of silicon oxynitride is 3.5um×3.5um, so as to realize the conversion of the mode field from the ridge waveguide to the single-mode optical fiber.

[0046] The end of the output unit is fused with the optical fiber coil through a pigtail. After the lithium niobate thin film waveguide core layer is prepared, a silicon dioxide film is deposited on the surface of the lithium niobate crystal as an upper cladding layer 10, and finally the chip end face is polished by chemical mechanical polishing (CMP). The complete process flow is as follows: Figure 5 shown.

[0047] The above process can complete the design and preparation of a thin-film lithium niobate integrated optical engine for fiber gyroscopes described in the present invention. In the first aspect of this embodiment, by combining gas-phase proton exchange with ridge waveguide, the advantages of each are brought into play, and the functions of polarization, coupling, beam splitting, phase modulation, etc. are integrated, which greatly reduces the size of the device, reduces the cost, and is suitable for more application scenarios; in the second aspect, the gas-phase proton exchange is similar to the input fiber mode field size, the coupling loss between the input fiber and the chip is very low, and the gas-phase proton exchange and ridge waveguide, ridge waveguide and output fiber have additional mode spot conversion structures, and the overall coupling loss is <1dB, and the insertion loss is <3dB.

[0048] The overall chip length is about 11-13mm, the size of the fiber optic gyroscope prototype can be reduced to 24-26mm, and the mass production index accuracy can reach 0.01° / h.

[0049] The embodiments of the present invention are described in detail above, but the contents described are only preferred embodiments of the present invention and cannot be considered to limit the scope of implementation of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of this patent.

Claims

1. A thin-film lithium niobate integrated optical engine for a fiber optic gyroscope, comprising: Substrate 01, lower cladding 02, lithium niobate thin film waveguide core layer 03, upper cladding 10; The substrate material is silicon, the lower cladding and upper cladding materials are both silicon dioxide, and the refractive index of the lower cladding is less than a preset refractive index threshold; The lithium niobate film is prepared by ion implantation and wafer bonding, and has a thickness of 600-800nm.

2. The thin-film lithium niobate integrated optical engine for a fiber optic gyroscope according to claim 1, characterized in that: The lithium niobate thin film waveguide core layer includes an incident unit 04, a polarizing unit 05, a coupling unit 0601, a beam splitting unit 0602, a mode spot conversion unit 07, a phase modulation unit 08, and an output unit 09; Both the input unit 04 and the polarizing unit 05 are gas-phase proton exchange waveguides, and naturally have polarizing functions, and can only support TE mode and TM mode transmission.

3. The coupling unit 0601 and the beam splitting unit 0602 according to claim 2 are characterized in that: The coupling unit 0601 and the beam splitting unit 0602 are integrated into one body, and the structure is 2×2MMI, and is prepared by gas phase proton exchange process. The light source is input from port 1, passes through the multi-mode interference region, and is output from ports 2 and 3, and the splitting ratio is 1:1; The 2×2 MMI is also used to combine the two signal lights after re-modulation to generate an interference signal, which is emitted from port 2 of the 2×2 MMI and coupled to a photodetector to realize signal monitoring.

4. The phase modulation unit according to claim 2, characterized in that: The phase modulation unit 08 includes a lithium niobate thin film ridge waveguide 0801 and a traveling wave electrode 0802. The lithium niobate thin film ridge waveguide 0801 is prepared on the lithium niobate thin film 03 by using an ICP etching method; The traveling wave electrodes 0802 are distributed on both sides of the ridge waveguide modulation arm 0801. The electrode material is gold, and it is a coplanar waveguide push-pull structure. The signal electrode is in the middle of the two modulation arms, and the ground electrodes are on both sides. When an electrical signal is applied, the two single-polarized lights are modulated to realize the electro-optical modulation function.

5. The pattern spot conversion unit according to claim 2, characterized in that: The mode spot conversion unit 07 is used for mode field matching between gas phase proton exchange and ridge waveguide, and adopts a double-layer conical structure. The material is silicon oxynitride, and the refractive index is regulated between the lithium niobate film and the upper and lower claddings. The reaction equation is: The refractive index of silicon oxynitride is regulated by controlling the ratio of N2O and SiH4, and it is prepared by inductively coupled plasma etching (ICP).

6. The emission unit 09 according to claim 2, characterized in that: The output unit 10 includes a spot conversion structure for coupling between the lithium niobate thin film ridge waveguide 0801 and the output optical fiber; the spot converter adopts a double-layer tapered gradient waveguide structure, and cooperates with the silicon oxynitride waveguide at the end to achieve the amplification of the spot size.

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